Electric furnace roof construction



Feb. 6, 1968 R. w. WOODRUFF' ET 3,367,292

v ELECTRIC FURNACE ROOF CONSTRUCTION Filed Feb. 9, 1966 INVENTOR. R065?W000RUFF 5w DAV/E5 HgZ United States Patent 3,367,292 ELECTRIC FURNACERGUF C(BNSTRUCTION Roger W. Woodruff, Bethel Park, and Ben Davies,Pittsburgh, Pa, assignors to Dresser Industries, Inc., Dallas, Tex., acorporation of Delaware Filed Feb. 9, 1966, Ser. No. 526,091 5 Claims.(Cl. 110-99) ABSTRACT OF THE DISCLOSURE- A basic electric furnace roofor a like dish-shaped dometype furnace roof substantially entirely madeof directbonded brick having a porosity over 20%, permeability of lessthan 1.5 units as hereinafter defined, a modulus of rupture at 2300 F.substantially twice that at room temperature and a density of not morethan 175 p.c.f.

This invention relates to fabrication of chemically basic electricfurnace roofs.

A good treatise on the subject of electric furnaces, the construction ofroofs therefor, and the like, is Electric Furnace Steelmaking, volume 1,Design, Operation, and Practice, published by Interscience Publishers, adivision of John Wiley & Sons, Inc., in 1962. It is a product of thePhysical Chemistry of Steelmaking Committee of The American Institute ofMining, Metallurgical, and Petroleum Engineers. The reader is directedthereto for a discussion of known prior art and, in particular, thesection entitled Roof Construction, beginning on page 163. As this workpoints out, silica brick and, to a lesser extent, certain types of firebrick and high alumina brick have been the conventional refractories forthe roofs of electric steelmaking furnaces. However, with increasinglysevere and extended furnace campaigns becoming conventional, such brickare proving of insufficient refractoriness. Basic brick would seem toprovide the logical substitute, but usage thereof has been consideredreplete with difficulties. First, the considerably increased density asone moves from silica (about 120 p.c.f.) to basic (180+ p.c.f.) brick soincreases the total weight of a given roof that much existing rooflifting and moving equipment is insufiicient.

Thus, it is an object of this invention to provide for basic electricfurnace roof construction more readily adaptable to existing electricfurnace shops.

Briefly, according to one aspect of this invention, there is provided anelectric furnace roof construction of chemically basic fired refractorybrick, which is relatively lightweight, but which has the necessaryelevated temperature strength for such a roof.

Other details and further features and advantages of this invention willbecome more readily apparent to those skilled in the art from a study ofthe following description together with reference to the attachedschematic drawing of an electric furnace. In these drawings:

FIG. 1 is a top view of a furnace roof according to the invention; and

FIG. 2 is a side sectional view along the line AA of the roof of FIG. 1.

Before describing the drawings in detail, it should be understood theyare but exemplary of preferred practice according to the concepts ofthis invention, and are provided for purposes of explanation and not byway of limitation. The true measure of the spirit and scope of theinvention is as defined in the hereafter appended claims.

In FIGS. 1 and 2, there is shown a plurality of concentric ringsarranged circumferentially within a circular roof band 20 to form adownwardly-opening dome-type dishshaped roof. Each of these rings 21 iscomprised of a plurality of burned, low-density, magnesia-chrome orerefractory brick as hereinafter defined. The first eleven "ice rings areentirely of brick. The remaining seven are incomplete, but substantiallyfill the space between the equispaced tubular electrode ports 22, 23,and 24. The remainder of the central area between the incomplete ringsand the electrode ports is fabricated of a ramming mix 25. Infabrication of the roof, a dome-shaped wooden form or the like is placedon the fioor or a comparable supporting surface and the concentric ringsare built up, the electrode ports constructed, and the ramming mixlikewise installed.

The shapes of the respective brick of each ring are dictated byconventional practice; for example, as shown on p. 507 of the bookModern Refractory Practice, 4th edition, published by theHarbison-Walker Refractories Company, in 1961.

When it was first determined that conventional basic refractory brickcould not be used successfully in a domeshaped electric furnace roof ofthe type allowing for free rise and fall without substantial distortion,and without special lifting equipment, it was suggested that means oflightening the overall weight of the roof would be a possible solution.To this end, batches of the type used to make brick of the typedisclosed and claimed in U.S. Patent No. 3,180,744 were studied. Thefirst thought was conventional burnout materials would be the bestsolution to provide lower density brick. The aim was to provide a brickhaving a density of less than about p.c.f. but, in any event,consistently less than about p.c.f. To this end, a plurality of batcheswere fabricated using as burnout material coarse coconut shell, ivorynut shell, and walnut shell. All of the mixes tested had approximatelyequal size gradation, the only difference from batch to batch being thetype of burnout material used. An exemplary batch would have about 10%of a 4+28 mesh burnout material. The batch using the ivory nut shellshad about 12% of that material, but also in the same 4+28 mesh sizerange. The remainder of the batch was suitably sizegraded to make anoverall satisfactory brickmaking sizegraded batch. The overall oxideanalysis of the batch was about 60-70% MgO, about 14-20% Cr O about 410%A1 0 less than about 3% SiO about 5-10% FeO, and up to about 3% CaO.

All of the brick batches were tempered with about 3.6% of a 50:50ligninzwater tempering agent, formed into brick on a conventional brickpress, and fired at a temperature of about 3050 F.

The mixes were all so designed as to provide a brick of about 150 p.c.f.or less when burned. The mix containing the ivory nut shells was badlydisrupted when ejected from the mold box. The mixes containing coconutor walnut shells showed slight pressure cracking and sponginess at thepress, and were disrupted when burning. It appeared the naturalelasticity of such burnout materials prevented their proper usage inmaking burned basic brick, particularly direct-bonded basic brick of thetype disclosed and claimed in the U.S. patent referred to above.

It was thought that coating the shells with refractory fines and usingsome of the tempering agent as a glue therefor would make the shellsless resilient. Indeed, the coated shells allowed brick to be pressedsatisfactorily. However, their high-temperature strength (i.e., modulusof rupture at 2300 F.) averaged about only 430 psi, and the highpermeability of 3.9 (allowing undesirable penetration by corrosivefurnace fumes) made them far from satisfactory. For this invention, thebrick should have an elevated temperature strength at least about twicethat at room temperature and a permeability (as hereinafter defined) ofno more than about 1.5.

Faced with the apparent total failure of conventional techniques formaking lightweight brick, other means had to be found to obtain thedesired low porosity but also obtain satisfactory elevated temperaturestrength. Thus,

31 in additional studies it was decided that the batch ingredientsthemselves should be so manufactured as to inherently have a highporosity (i.e., be lightweight) and yet not lead to undesirablepermeability. The following units. In the practice reported herein, thepermeability value represents the volume of dry air, measured at roomtemperature and atmospheric pressure, which will flow through a one-inchcube of the test material in one sectable illustrates the results of theadditional testing. ond under a pressure differential of one p.s.1. Theun1t 1s TABLE I A B C D E Power Pressed at 8,000 psi. Impact PressedSeconds 4 Hammers Standard 1 Light Mag- Light Mag- Standard with LightAll Light nesite With nesite With Magnesite Magnesite Special FinesSpecial Fines Mix (percent):

Magnesite,4/10 Magnesite, 10/28. 1O Magnesite, Ball M 1F e (5560% 30 33.1 44 Light Magnesite, 4/10 25. 7 2e 2e 30 Light Magnesite, 10/28 8. 6 3030 Philippine Chrome Ore Concentrates, 6/28 6 6. 6 Philippine Chrome OreConcentrates, -28 24 26. 3 Grain of Type Disclosed in US. Patent No.3,116,156,

Ball Mill Fines (55-60% 325 Mesh) 44 Grain of Type Disclosed in 11.8.Patent No. 3,116,156, 3 Mierons (Fisher Subsieve Analyzer AverageDiameter) 35 Burn, Temperature, F. (lil'Hour Hold). 3, 050 3, 050 3, 0503, 000 Bulk Density, [1.0.1. (Av. 4) 174 153 172 172 Modulus of Rupture,p.s.i.:

At Room Temperature 500 430 1,840 1, 020 At 2,300 F. (hold time 5 hours)960 910 140 2 1, 090 2 1, 180 Apparent Porosity, percent 18. 4 23. 5 30.4 30. 2 25. 6 Permeability 3 l- 4 1. 3 0. 47 0. 46 1. 4

1 According to United States Patent No. 3,180,744. 2 Breaking capacityof equipment exceeded at this point. a As hereinafter defined.

The testing provided rather surprising results. Superior brick requiredboth careful control of constituents as well as manufacturing procedure.Mix A (according to US. Patent No. 3,180,744), as compared to Mix B, wassubstantially identical except that light magnesite was used,

and showed considerably lesser density, but equivalent (withinexperimental error) elevated temperature strength. Brick entirely of thelight magnesitc (Mix C) were totally unsatisfactory at elevatedtemperatures. The light magnesite mixed with fused grain (of the typedisclosed in US. Patent No. 3,116,156) and manufactured into brick(according to teachings of Patent No. 3,199,994) provided even lowerdensity brick having elevated temperature strength which exceeded thebreaking capacity of the equipment being used. Mix D is the preferredembodiment of the invention having as it does the lowest permeabilitywith excellent high-temperature strength. Another very desirablecharacteristic of the brick illustrated by the data is that, despitevery high porosity or void area, the permeability (a measure ofinterconnected void or pore area) is very low. This is, in large part,due to the use of lightweight rnagnesite grain in which contained porespace is substantially discontinuous. Preferably, the porosity is over30% and the permeability less than about 0.5%. It is satisfactory thatthe porosity be above about 20% and the permeability less than about1.5%.

It is preferred the lightweight, low-density magnesite constitutesubstantially the coarser 4+ 28 mesh fraction of the brickmaking batchto assure the maximum amount of contained encapsulated or disconnectedpore space therewithin is notbroken by crushing too fine in making asized batch.

From the foregoing tests, permeability was determined substantially asdescribed in the article by G. R. Eusner and I. T. Shapland inPermeability Of BlastFurnace Refractories, Journal of the AmericanCeramic Society, vol. 42, No. 10, pp. 459464, 1959. In the practice ofthis invention, the permeability may be expressed in various stated ascu. in./sec./sq. in. of area/in. thickness/psi. pressure. The followingformula is used:

Permeability flow rate (ca/min.) thickness (in.) X000526 area (in?)pressure (cm. of Hg) Thus, essentially, the present invention is basedupon fabricating the major portion of a roof of an electric furnace withburned direct-bonded basic refractory shapes having low density butexcellent high-temperature strength. This is, in large part, predicatedupon using a magnesite having an inherent density of no more than about2.70. The magnesite must be such that the permeability of the resultingshape is less than about 1.5 permeability units, as above defined. Themagnesite and chrome spinel constituents of the burned shapes must becharacterized by substantial direct-bonding substantially withoutintervening silicate filming. The modulus of rupture of the shapes at2300 F. should be at least about twice that at room temperature. Ofcourse, to properly make such brick the total SiO content of the batchshould be maintained at less than about 2%.

The chrome ore used in the tests reported had the following typicalchemical analysis:

Percent Silica, (SiO 2.4 Alumina, (A1 0 33.4 Iron Oxide, (FeO) 11.2Chromic Oxide, (Cr O 35.0 Lime, (CaO) 0.3 Magnesia, (MgO) 17.1

All magnesite (both dense and lightweight) had the following typicalanalysis:

Silica, (SiO 0.7 Alumina, (A1 0 0.3 Iron Oxide, (F0 0 0.3 Lime, (CaO)0.9 Magnesia, (MgO) 97.5 Loss on Ignition 0.1

In the foregoing discussion, all parts and percentages are by Weight,analyses are on an oxide basis, and sizing refers to the Tylerseries-unless specifically stated to the contrary.

Having thus described the invention in detail and with suflicientparticularity as to enable those skilled in the art to practice it, whatis desired to have protected by Letters Patent is set forth in thefollowing claims:

We claim:

1. In an electric furnace roof or the like having a downwardly-openingdish-shaped dome-type roof, said roof being fabricated of a plurality ofceramically bonded basic refractory shapes contained in adjacentconcentric rings within a roof band, substantially all of the brick insaid roof being direct-bonded brick having a porosityover about 20% buta permeability of less than about 1.5 units as determined by theformula:

Permeability flow rate (co/min.) Xthiekness (in.) 0.00526 area (in?)pressure (cm. of Hg) 6 rality of brick in said roof being no more thanabout 175 p.c.f.

2. The electric furnace roof construction of claim 1 in which thedensity of said brick is less than about 150 p.c.f.

3. The electric furnace roof construction of claim 1 in which theporosity of said brick is above about 30% and the permeability belowabout 0.5.

4. The electric furnace roof construction of claim 1 in which said brickare direct-bonded brick made from a sizegraded batch of dead-burnedlow-density lightweight magnesite and low-silica chrome ore.

5. The electric furnace roof construction of claim 4 in which thelightweight magnesite forms substantially all the coarser -4-|-28 meshfraction of the batch.

References Cited UNITED STATES PATENTS 2,814,476 11/1957 Leitner 263463,180,744 4/1965 Davies et al. 106--59 3,194,672 7/1965 Davies et al.10659 3,309,175 3/1967 Berg et al. 10658 FREDERICK L. MATTESON, JR.,Primary Examiner.

J. J. CAMBY, Assistant Examiner.

